runtime: refactor obtaining the system stack

The system stack is only needed when we're not on it. So we can directly
call task.SystemStack() without problems.

This also saves a tiny bit of binary size.

builder/sizes_test.go

@@ -42,9 +42,9 @@ func TestBinarySize(t *testing.T) {
 	// This is a small number of very diverse targets that we want to test.
 	tests := []sizeTest{
 		// microcontrollers
-		{"hifive1b", "examples/echo", 4560, 280, 0, 2268},
-		{"microbit", "examples/serial", 2924, 388, 8, 2272},
-		{"wioterminal", "examples/pininterrupt", 7383, 1489, 116, 6912},
+		{"hifive1b", "examples/echo", 4556, 280, 0, 2268},
+		{"microbit", "examples/serial", 2920, 388, 8, 2272},
+		{"wioterminal", "examples/pininterrupt", 7379, 1489, 116, 6912},
 
 		// TODO: also check wasm. Right now this is difficult, because
 		// wasm binaries are run through wasm-opt and therefore the